A known method of testing open but not completely visible cavities is to take x-ray pictures of the workpieces with the cavities to be tested. But this method requires expensive equipment and is extremely complex.
In addition, it would be feasible to determine the flow resistance of the cavities and to compare it with a standard value. This is, however, only possible if the cavities have two entries so that a flow medium can be directed into one entry and carried off by the other. In addition, flow resistance measurements are very complex since they require a sealed connection of the cavity with the measuring apparatus. Finally, measurable flow resistance is only rarely a criterion for the correct shape of the cavities.